How to Add Gradient Texture to My Node Setup?
You cannot directly mix different materials.
Turn one of them into a node group, and it will work.


Turn Material 01 into a Node group
Add the new Node group into Material 02 using a mix shader
Connect Gradient texture and use ColorRamp to control the gradient


Create a nodegroup from one of your materials.



Add the new node group into the other material using a mix node.
Use ColorRamp to control the gradient.



Notes:
Gradient texture connects into the FAC input of a mix node (Mix RGB, Mix Shader or ColorRamp)
To make the gradient vertical, I used a UV map projected from view.